How does the mix shader work when using a Fresnel node as a factor?
The fresnel node will produce a mask like this:



when you use it as a factor you are just saying that the first input will replace the black and the 2nd will replace the white,

when you switch the inputs,you are just saying that the black should be the 2nd input and the white should be he first.

You can see how this work  using a mix color node to visualize it:


By switching :